The matrix \(\mathsf {A}\)  corresponds to the divergence-conforming
HDG discretization of the elliptic operator \(\frac{2}{\delta t}\rho \hbox{$u$}-2\nabla \cdot (\mu \mathbf {D}(\hbox{$u$}))\) . Here we propose to use
the auxiliary space precondioner [1]} developed in [2]}.
The auxiliary space is the continuous linear Lagrange
finite elements:
\(\hbox{$V$}_{h,0}^{cg} := \lbrace \hbox{$v$}\in \hbox{$H$}_0^{1}(\Omega ):\;\;\hbox{$v$}|_K\in [\mathcal {P}^1(K)]^d, \;\forall K\in \mathcal {T}_h\rbrace .\) 
